Program Director/Principal Investigator (Last, First, Middle): Kalivas, Peter W PROJECT SUMMARY (See instructions): Addiction to opioids is a poorly treated disorder that has reached epidemic proportions in the USA. The proposed supplement will add experiments utilizing heroin self-administration and cued heroin seeking to an existing RO1 grant (2 RO1 DA012513) whose specific aims are focused entirely on cocaine-induced plasticity in cortico-accumbens and accumbens-ventral pallidal synapses. Here we describe two supplemental aims to evaluate the impact of heroin withdrawal and cued heroin seeking on synaptic plasticity in the GABAergic projection from the core of the nucleus accumbens (NAcore) to the dorsolateral ventral pallidum (dlVP). In Supplemental Aim 1 we use optical stimulation techniques and whole cell patch clamping of dlVP neurons to examine heroin induced synaptic plasticity in D1- and D2-medium spiny neuron (MSN) inputs. Both inputs express mu opioid receptors (MORs) and D2 inputs undergo marked long-lasting changes in MOR regulation of GABAergic transmission after withdrawal from cocaine. Given that MORs are the direct pharmacological targets of addictive opioids and these data generated in the parent grant showing MOR regulation of GABAergic plasticity in the dlVP, we hypothesize that heroin self-administration will markedly alter D2 and possibly D1 GABAergic afferents. Moreover, we will expose rats to heroin associated cues to reinstate heroin seeking. Based on previous work showing transient plasticity in D1 MSNs in the NAcore by cued drug seeking, we expect transient plasticity will occur preferentially in D1 MSN GABAergic after cued reinstatement. Supplemental Aim 2 builds off of our recent preliminary data showing marked morphological changes in astroglia in the NAcore after heroin withdrawal and during heroin cue-induced drug seeking. In this aim we conduct exploratory experiments to determine if astroglia in the dlVP also undergo astroglial morphological adaptations after heroin withdrawal or cued reinstatement. These two supplemental Aims will establish the data infrastructure for substantial future research to understand not only the actions of heroin in the VP to promote relapse vulnerability, but studies into the basic biology of how astroglia regulate synaptic transmission.